A safety syringe of the prior art emphasizes the function that the needle thereof will be pulled back into the syringe barrel by a link rod after being used. However, such a structure cannot destroy the needle completely.
A safety syringe of rotational contraction of the prior art can attain the goal of single usage; but it is quiet costly to manufacture and cannot be immune from the possibility of being used again. The safety syringe of rotational contraction has a hook portion in the front of a needle barrel for forming a granular support against a needle hub. A receptacle is formed at the rear end of the needle hub, which receptacle has a small neck portion, whereby a conic head in the front of a push rod will be locked within the receptacle. The conic surface of the head is further provided with a plurality of projected teeth for engaging corresponding holes on the inner wall of the receptacle, whereby a user may rotate the needle hub so that a plurality of granules on the hub will be departed from the hook portion. Thereby, the needle hub can be pulled back into the barrel, and an oblique flexible rod at the front end of the conic head will tilt the needle hub and the needle into an oblique configuration and therefore cannot be used again.
The safety syringe of rotational contraction of the prior art has the following disadvantages. The hook stricture for preventing a departure of the needle hub from the barrel is locked in a groove of granular structure, which is quite a complex design. It is further complicated by the engagement between the projected teeth on the conic head and the holes within the hub. Such a complicated design is difficult to make by plastic injection molding.
Further, as the flexible rod is entering, the attack angle of the rod should be controlled with precision so as to avid hitting the inner wall of the barrel and gets deflected. To achieve the goal of tilt the needle hub the projected teeth on the conic head should be precisely coupled with the holes of the hub, which is too complex a structure.